It depends on how you are currently storing your collection, dimensions of you storage area, and are you willing to change your storage methodology. We have a consolidated storage area in the Washington D. C. area and they love a tier rack (http://www.tier-rack.com/). They are flexible and stackable to as high as you are comfortable or you can go. The other thing I have seen with an storage area in a warehouse space with high ceilings at Yosemite National Park is a steel platform (website is an example of what I am talking about: http://www.schmidtstructural.com/). They were able to place wardrobe cabinets on the concrete floor as well as on this platform, and the company they went through also built them an elevator to access the second level with heavy objects. The last option I have seen used at Badlands National park is compactor storage. This can be designed to fit your current cabinets and expand your existing floor space.
We are currently looking at retrofitting a historic structure but are fighting an uphill battle with budget issues. With the way funding is for cultural institutions, I would personally retrofit the current structure if you can, it is cheaper and if there is nothing wrong with the old storage location there are many creative ways of expanding space.
Hope this helps